A transistor using a semiconductor film formed over an insulating surface is an essential semiconductor element for a semiconductor device. Since the manufacturing of transistors has a limitation on the allowable temperature limit of a substrate, a transistor using, as an active layer, amorphous silicon which can be formed at a relatively low temperature, polysilicon which can be obtained by crystallization using laser light or a catalytic element, or the like has been mainly used as a transistor for the semiconductor display device.
In recent years, a metal oxide having semiconductor characteristics which is referred to as an oxide semiconductor has attracted attention as a novel semiconductor material which has both high mobility, which is a characteristic of polysilicon, and uniform element characteristics, which is a characteristic of amorphous silicon. The metal oxide has been used for various applications; for example, indium oxide that is a well-known metal oxide has been used as a material of a transparent electrode included in a liquid crystal display device or the like. Examples of such metal oxides having semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, and zinc oxide. Transistors in each of which a channel formation region is formed using such a metal oxide having semiconductor characteristics have been known (Patent Documents 1 and 2).